borgbackup/docs/_templates/versionselector.html

<div class="version-selector" id="borg-version-selector" style="display:none;">
  <label for="version-select">Select your Borg version:</label>
  <select id="version-select"></select>
</div>
<script type="text/javascript">
  // Populate the version selector using ReadTheDocs data if available.
  function borgInitVersionSelector(data) {
    var versions = data && data.versions && data.versions.active;
    if (!versions || !versions.length) return;
    var current = data.versions && data.versions.current && data.versions.current.slug;
    var select = document.getElementById("version-select");
    if (!select) return;
    versions.forEach(function(v) {
      var opt = document.createElement("option");
      opt.value = v.urls.documentation;
      opt.textContent = v.slug;
      if (v.slug === current) opt.selected = true;
      select.appendChild(opt);
    });
    select.addEventListener("change", function() {
      window.location.href = this.value;
    });
    document.getElementById("borg-version-selector").style.display = "";
  }

  document.addEventListener("readthedocs-addons-data-ready", function(event) {
    borgInitVersionSelector(event.detail.data());
  });
</script>